Pseudomonas aeruginosa is a microorganism which occurs ubiquitously and is regarded as a "problem microbe" in human medicine. Its prime target is debilitated patients and it can frequently be controlled by antibiotic therapy only with difficulty. At particular risk are patients in intensive care units and paraplegics as well as people who have suffered burns or are exposed to an increased risk of burns, such as firemen or steelworkers. In all these cases there is the prospect of preventing infection by active immunization. Lipoprotein I (OMPI) is, like porin F (OMPF), a major constituent of the outer membrane of P. aeruginosa and, for this reason, is likewise potentially suitable as constituent of a vaccine.